Ethernet and VLAN overlapping

[m4rlung]

Hi,

So I have a FastEthernet on my 2811 router which has 192.168.1.130 / 30 and DHCP pool which is in network 192.168.1.168 / 26. When I try to configure VLAN 10 which should have 192.168.1.169 / 26 it says that it overlaps with FastEthernet 0/0.... I don't understand the range going down to 192.168.1.128 I thought that it should go from 168 to 223 (broadcast).

Thanks for any advice

[MHM Cisco World]

there is overlapping 
check below 
MHM

[Richard Burts · Accepted solution]

If you are dealing with subnets using /26 then there are 64 addresses in each block. And the blocks begin at 0, 64, 128, and 192. If you configure network 192.168.1.168 / 26 it does not mean that the subnet necessarily starts at 168. You have picked an address in the middle of the block, and the actual subnet for that address starts at 128, and so there is indeed overlap.

[M02@rt37 · Accepted solution]

Hello @m4rlung 

The subnet 192.168.1.128/30 has a range from 192.168.1.128 to 192.168.1.131, and it only accommodates two usable IP addresses: 192.168.1.129 (router's FastEthernet interface) and 192.168.1.130 (other device).

Now, if you want to create a VLAN (let's say VLAN 10) with a subnet of 192.168.1.168/26, this range includes IP addresses from 192.168.1.128 to 192.168.1.191. This overlaps with the existing subnet on FastEthernet0/0.

To avoid conflicts, you should choose a non-overlapping IP range for your VLAN. For instance, you could use a subnet like 192.168.1.192/26 or any other range that doesn't overlap with your existing subnet on FastEthernet0/0.

Make sure that when you define VLANs and subnets, each subnet is unique across your network to prevent IP conflicts and segmentation issues.

[m4rlung]

Thank you all for answers.

[m4rlung]

And also M02@rt37 @Richard Burts @MHM Cisco World how would you proceed to make a subnet of 62 hosts in range which cannot overlap 192.168.1.128 and 192.168.1.224? I am trying to make one subnet manageable by one VLAN if it's even possible. 

[Richard Burts]

In my response I made a point that if you have some subnets using /26 then there is room for 4 subnets in 192.168.1.0 that do not overlap/duplicate. So if you have something in 128 and in 192 (which includes 224) then you could create the new subnet using either 0 or 64. So it is certainly possible to create another subnet with 62 hosts.

I have the impression that in your concept of subnets you think you can pick a starting address and then get x number of hosts (64 for /26) starting from that point. It does not work that way. For /26 subnets there are specific addresses that are the beginning of the subnets (0, 64, 128, 192 for /26).